To keep on track with the famous Moore's law, the next generation of EUV (Extreme UV) lithography has been committed to by major industrial players. Generating EUV is a complicated and inefficient process. For instance, 50 W EUV is currently generated using 20 kW infrared (IR) radiation at a wavelength of 10.6 μm. Efficiencies are expected to improve by up to 50% (i.e. 75 W EUV), but this still won't meet target values of up to 250 W or more. This means that even higher IR powers will be needed (40-50 kW) putting existing optical laser components such as mirrors, windows, and lenses under even more thermal stress. Typical optical systems for next generation EUV lithography comprise many mirrors (tens of mirrors per system) and typically each mirror has a diameter of 50 mm or greater.
To date, coated copper mirrors, such as gold coated copper mirrors, have been utilized, which are adequate for demonstrator systems but unlikely to be adequate for production systems. As such, there is a need to provide new mirror components which are suitable for use in such high power laser systems. The basic technical needs for such mirrors include the following:                High flatness        High stiffness        Large areas (>50 mm)        Low weight/density—since the mirrors need to be adjusted in adaptive optics systems, the density/weight of copper is an issue        Optical tolerances of the order of lambda/10 or better where lambda is 10.6 μm        Low coefficient of thermal expansion (CTE) to avoid thermal lensing type effects        High thermal conductivity        High laser induced damage threshold        
Claude A. Klein, “High-Power Laser-Mirror Faceplate Materials: Figures of Merit for Optical Distortion”, SPIE Vol. 3151 discusses problems of thermal lensing in laser mirrors. Potential mirror-face plate materials are assessed from a theoretical stand-point using material characteristics to generate a figure of merit as a gauge for comparing thermal lensing performance of mirror material candidates in a pulsed or continuous wave laser environment. The figure of merit calculations indicate the following rating of potential laser mirror materials listed from worst to best: copper; molybdenum; silicon; silicon carbide; carbon-carbon (carbon fibre reinforced graphite); and diamond. It is indicated that copper still plays an important role as a mirror material for industrial CO2-lasers but does not match the performance of other substrate materials when thermal lensing becomes an issue. Molybdenum exhibits a combination of physical properties that make it more attractive than copper, e.g. a lower thermal CTE, and is also easy to machine and polish. The two ceramics, silicon and silicon carbide, are good candidates for high energy laser applications. Finally, it is indicated that polycrystalline diamond and carbon-carbon composites are both outstanding candidates based on their thermal properties. However, there have been difficulties in adapting carbon-carbon composite fabrication techniques to cooled mirror configurations. Furthermore, while diamond exhibits particular promise for high-heat-load optics applications that require efficient cooling, there are some problems with using diamond for high energy laser mirrors. For example, large area diamond components are expensive to manufacture. Furthermore, while polycrystalline chemical vapour deposited (CVD) diamond material has the advantages of being very hard and stiff with a high thermal conductivity and a low thermal expansion coefficient, the material has relatively low toughness and is difficult to process to the high precision surface finishes required for mirror applications. Further still, it can be difficult to reliably bond reflective coatings to diamond substrate materials, particularly when components are exposed to high power lasers with thermal expansion coefficient mismatches leading to delamination problems. In addition, it is difficult to form polycrystalline CVD diamond to high thicknesses and/or into three-dimensional shapes, e.g. when curved mirrors are required.
It is an aim of embodiments of the present invention to address one or more of the aforementioned problems.